This invention relates to a process for synthesizing complete mixed polyol esters, that is, polyol esters having at least two different ester groups and no hydroxyl groups. More particularly, this invention relates to a process for esterifying partial polyol esters without rearrangement of ester groups either by intermolecular or intramolecular acyl group exchange. The term "partial polyol ester" is used herein to denote a polyol which is partially, that is, incompletely esterified and as a consequence contains at least one hydroxyl group.
In general, this process provides mixed polyol esters with specific ester groups at specific polyol hydroxyl sites. Thus, this process is especially useful for providing synthetic cocoa butter and closely related oleaginous substitutes from inexpensive raw materials such as lard and palm oil.
Cocoa butter is unusual among naturally occurring fats in that it is normally a brittle solid up to 77.degree. F., has a relatively narrow melting range and is almost completely liquid at 95.degree. F., or slightly below body temperature. These unique melting characteristics make cocoa butter suitable for use in confectionery products, especially chocolates. Such melting characteristics contribute glossy coatings, absence of stickiness and favorable volume changes during confectionery product molding.
Because of these advantageous melting characteristics and because of the demand for the properties which cocoa butter imparts to confectionery products, large quantities of this expensive commodity are imported even when domestic fats which can be used to produce synthetic cocoa butter are in plentiful supply at much less than the cost of cocoa butter. For many years, therefore, attempts have been made to provide from readily available and cheaper fats a product that can be used to replace at least part of the cocoa butter in chocolates and other confectionery products that normally contain cocoa butter.
In this search for a synthetic cocoa butter, it has been determined that its advantageous physical characteristics are derived from the arrangement of the fatty acid substituents in its glycerides. Analytical tests have shown that cocoa butter comprises principally 1-palmitoyl-2-oleoyl-3-stearoyl glycerol, and minor amounts of triglycerides having a different order of substitution of the palmitoyl, oleoyl and stearoyl groups on the glycerol molecule. Accordingly, 1-palmitoyl-2-oleoyl-3-stearoyl glycerol would provide the desired cocoa butter substitute, were this compound readily available.
With most esterification procedures, the synthesis of such substantially pure specific triglycerides is impossible since substantial ester group rearrangement occurs during esterification of specific partial glycerides, namely, mono- and diglycerides, the synthesis of which is known in the prior art. Thus, acylation of 1,3-diglycerides with oleic acid and a conventional acid esterification catalyst provides only a minor proportion of triglycerides having an oleoyl group at the 2-position, where this group must necessarily occur to provide the desired synthetic cocoa butter.
Feuge, Willich and Guice, the Journal of the American Oil Chemists Society, July 1963, pp. 260-264, demonstrate that ester group rearrangement ordinarily occurs during the esterification of partial glycerides, and, at page 260, point out that hydrochloric, sulfuric and hydrocarbyl sulfonic acids, which are widely used as esterification catalysts, cause ester group rearrangement. Accordingly, these acid catalysts are not suitable for preparing the desired position-specific (i.e., 2-oleoyl) triglycerides for use as a cocoa butter substitute. Similarly, ester group rearrangement ordinarily occurs during esterification of partial polyol esters other than glycerides, e.g., during esterification of partial 1,2-propylene glycol esters.
One known method for synthesizing a cocoa butter substitute comprises reacting a diglyceride having palmitoyl and stearoyl groups at the 1- and 3-positions with oleoyl chloride; see U.S. Pat. 3,012,890. Furthermore, it is known in the prior art that, in general, acid chlorides can be used to specifically esterify mono- and diglycerides. The use of acid chlorides for specific esterifications has many undesirable aspects, however. For instance, acid chlorides are very corrosive and their use involves handling problems. Besides, hydrochloric acid, a by-product of the reaction of an acid chloride with a hydroxyl group, is difficult to remove from the oleaginous reaction product, a critical factor inasmuch as the product is to be used as a food.
U.S. Pats. 3,410,881 and 3,337,596 disclose the use of perchloric acid as an effective catalyst for preparing a cocoa butter substitute without rearrangement of the ester groups. However, perchloric acid is known to be explosive and its use in the presence of organic compounds is preferably avoided.
It has now been found that perfluoroalkyl sulfonic acids catalyze the esterification of partial polyol esters without ester group rearrangement. Gramstad et al., J. Chem. Soc., 4069, 1957, teach the use of trifluoromethane sulfonic acid as an esterification catalyst but do not suggest that it would be useful in the preparation of triglycerides without ester rearrangement. Moreover, the previously noted Feuge et al., article suggests that sulfonic acid esterification catalysts per se cause group rearrangement.
It is therefore an object of this invention to provide an improved process for synthesizing specific complete mixed polyol esters, especially specific mixed triglycerides. It is a further object to provide a process for synthesizing specific complete mixed polyol esters with substantially no rearrangement of ester groups either by intermolecular or intramolecular exchange. It is a further object herein to provide a process for synthesizing specific complete mixed polyol esters without using perchloric acid or acid chlorides. Yet another object of this invention is to provide a process for the preparation of synthetic cocoa butter. These and other objects are obtained herein as will be seen from the following disclosure.